1. Field of the Invention
The present invention relates to an impregnated soft substance gasket preferably composed of a metal-reinforced fiber mat, such as an impregnated cylinder head gasket, an exhaust flange gasket or an ancillary seal for internal-combustion engines. This invention also relates to an impregnating agent based on a cross-linkable silicone for impregnating the gasket.
2. Description of the Background
Soft substance cylinder head gaskets for internal-combustion engines are customarily impregnated. This is done primarily to improve their service life and sealing quality as well as to increase their strength.
According to U.S. Pat. No. 3,970,322, such impregnating agents are composed of liquid polybutadiene, polyacrylates and compositions including isocyanates or epoxides. The impregnating agents are preferably thermally cross-linked in the impregnated sealing plate, possibly with the addition of cross-linking agents.
Liquid silicone systems are also known which cross-link into silicones having primarily good resistance to the media to be sealed, to the influence of temperatures and to aging. This is preferably attained with the addition of metal salt catalysts. Such cross-linkable liquid silicones can also be used to impregnate soft substance cylinder head gaskets.
According to DE-OS No. 3,245,664, liquid silicones capable of polyaddition can serve as impregnating agents. Examples of these silicones can be cited such as a polymethyl vinyl siloxane which cross-links after the addition of a platinum complex compound. However, these platinum complexes are poisoned primarily by amine or sulfur compounds and thus lose their effectiveness.
According to DE-OS No. 3,245,664, fiber mat seals can contain rubber binders which are free of amines and sulfur so as to permit the impregnating agent to be sufficiently cross-linked. Such fiber mat seals, however, are relatively expensive to manufacture for special seals.
It is also expensive to produoe fiber mat materials which are either free of asbestos or if the mat materials contain asbestos, the asbestos are free of amines or sulfur compounds and other catalyst poisons.
According to DE-OS No. 3,317,501, polysiloxane systems containing reactive and cross-linkable hydrogen groups such as methyl hydrogen polysiloxanes are also used to impregnate soft substance cylinder head gaskets. The cross-linking in the impregnated seal then takes place primarily by using compounds such as organic tin, aluminum or lead salts. However, even after the cross-linking, the cross-linked siloxanes still contain a percentage of polysiloxane having unbound and reactive hydrogen groups which may then continue to react in the finished seal.
Primarily occurring in the surface of the seal, such postreactions may occur with the materials with which they are in contact, and the seal then becomes tacky and in an undesirable fashion sticks to the packaging material.
Another particular drawback which occurs in a cylinder head gasket is that after assembly the cylinder head gasket sticks to the metal sealing faces of the engine or to the cylinder head.
According to DE-OS No. 3,317,501, such a seal can therefore be subsequently impregnated or coated in a second process step with a condensation cross-linking polysiloxane resin. Such requirements in a second process step are uneconomical, and coatings over the entire surface area of a seal are usually not necessary at all to assure proper functioning.